The present invention relates to a method of manufacturing a device for displaying the function of a switch or other operable device, for example, switches used in controls for water pipes, gas pipes, or electric circuits.
Recently, improved devices for displaying the function of a switch used in the controls for water pipes have been developed to improve visibility and reading of its displayed letters or symbols, with improved appearance and resistance to soiling or peeling. In these improved display devices, a cavity is formed as part of a metal body and a sheet having the necessary letters, symbols or figure is placed on the bottom face of the cavity. The cavity is then filled with a transparent synthetic resin and cured. However, the paper used for the sheet in that display device may not contact the bottom face of the cavity evenly due to warping of the paper. This results in a distorted image of the display when viewed through the transparent synthetic resin. Where a stiff sheet which does not produce warp is employed for a display sheet, air remains between the sheet and the bottom face of the cavity and bubbles may later appear in the cured resin, which spoils the appearance of the display. If a sheet precoated with adhesive on its reverse side is employed, it is difficult to set the sheet in position in the manufacturing process because the cavity is small. Moreover, adhesives will produce stripes in cured resin due to the differences between an adhesive and a resin.
Disclosures of previous devices do not describe any specific transparent resin. However, a suitable resin should have certain physical mechanical properties. For example, the resin should be of sufficiently low viscosity to provide workability and yet avoid confinement of air bubbles. It should also be of sufficiently high viscosity when in a paste state that it will rise in the middle to produce a convex lens effect when in the cured state. The resin also should not produce a crease on the surface when curing, and should not be curable at normal (room) temperature. When the resin is heated, the curing speed should still be low enough to disperse air, while on the other the resin should cure rapidly enough, at a low enough temperature as not to deteriorate the display sheet. The resin must therefore meet various requirements which seem incompatible, or are inconsistent with each other. In addition, the resin should have good transparency, heat resistance, chemical resistance, water resistance and weather resistance to prevent deterioration such as yellowing with the passage of time and should be hard enough not to be easily marred such as by fingernails.